Natural gas from many gas fields, which is often produced at high pressures, possibly as high as 50 MPa, can contain significant levels of H2O, H2S, CO2, N2, mercaptans, and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gases, H2S, and CO2 be removed from natural gas as possible to leave methane as the recovered component. Small increases in recovery of this light component can result in significant improvements in process economics and also serve to prevent unwanted resource loss. It is desirable to recover more than 80 vol %, preferably more than 90 vol %, of the methane when detrimental impurities are removed. In many instances effective removal of the H2S is more important than CO2 removal as specifications for natural gas transmission pipelines typically limit the H2S content to be as low as 4 vppm while a more relaxed specification of two to three percent is typically permissible for CO2. If the contaminant removal process is unselective between these two gases or favorable to CO2 removal, the treatment will be unnecessarily severe, resulting in increased processing costs. A natural gas treatment process which is selective for H2S relative to CO2 is therefore economically attractive.
Methane may also be recovered from methane hydrate formations. A large volume of methane is currently contained in permafrost regions in the form of methane hydrates. In many cases, it may be desirable to recover the methane from the methane hydrates. Methane gas can be recovered from a methane hydrate formation by adding a gas mixture containing nitrogen and carbon dioxide gases to the methane hydrates. Specifically, the methane within the methane hydrates is reacted with the gas mixture, and the gas mixture replaces the methane within the methane hydrates, thus releasing the methane. The gas mixture will typically contain CO2 and N2. The CO2 and N2 may be produced from treatment of refinery flue gas through a swing adsorption process such as those described in U.S. Provisional Application No. 62/256,383, which is incorporated herein by reference. The CO2 within the gas mixture replaces the methane within the methane hydrates. In addition, the N2 within the gas mixture aids in the release of the methane from the methane hydrates by increasing the temperature of the methane hydrates. In such applications, however, the CO2 and N2 must be injected into the hydrate formation at higher pressures, such as at least 25 bar, or at least 30 bar.
Additionally, Carbon Capture and Sequestration (CCS) is at the forefront of the energy industry. CCS generally encompasses the field of capturing waste carbon dioxide from large point sources, such as refineries or coal fired power plants, transporting it to a storage site and depositing it where it will not enter the atmosphere, such as an underground geological formation.
Synthesis gas or “syngas” is a byproduct a variety of refinery processes. Syngas is a mixture comprising carbon monoxide, carbon dioxide, and hydrogen. It is produced by gasification (or burning/combustion) of a carbon containing fuel to a gaseous product. Production of syngas is ubiquitous to refinery processing via the inevitable use of furnaces, boilers, reformers and the like found in hydrocarbon processing. Even in emerging technologies, such as fuel cells, syngas is produced as a byproduct along with electricity, water, and heat.
The Water Gas Shift (WGS) reaction is an important player in CCS and the proper handling of syngas. WGS describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen.CO+H2OCO2+H2 
As can be see, the WGS reaction provides a source of hydrogen at the expense of carbon monoxide. Hydrogen is a valuable product and can be used in hydroprocessing applications, which generally refers to conversion of heavy petroleum fractions into lighter ones via hydrocracking. It can also be used to produce ammonia. Hydrogen used in hydroprocessing applications must be extremely pure to be effective for its intended purpose, e.g. 99.9% pure, or 99.99% pure.
Hydrogen is most abundantly produced by steam methane reformers (SMR) in petrochemical facilities. Steam reforming describes the reaction of methane with steam to produce hydrogen and carbon monoxide.CH4+H2O→CO+3H2 Here, methane is exposed to steam at very high temperatures to form carbon monoxide and hydrogen. In a second stage, additional hydrogen is produced by exposing the carbon monoxide product to the WGS reaction described above.
Sorption Enhanced Water Gas Shift (SEWGS) describes processes where the WGS reaction is combined with CO2 capture. Syngas enters the SEWGS unit where carbon monoxide is treated with steam to produce carbon dioxide and hydrogen. The carbon dioxide is then adsorbed onto an adsorbent producing a nearly pure hydrogen product. Carbon dioxide can then be desorbed and then deposited via the sites CCS facilities.
Conventional SEWGS methods for capturing carbon dioxide tend to reduce the efficiency of the CCS process, due to the additional steam energy required to capture and/or sequester the carbon dioxide. Specifically, conventional processes utilize a costly, energy intensive steam rinse, which creates a large energy penalty on the plant. There is a need for an adsorption module and process that can produce pure hydrogen in an economical way.
Natural gas or syngas treating is often carried out using solid sorbents such as activated charcoal, silica gel, activated alumina, or various zeolites. The well-established pressure swing adsorption (PSA) process has been used in this way since about the 1960s. In the PSA process, the solid sorbent is contained in a vessel and adsorbs a gas species at high pressure and when the design sorption capacity of the sorbent is attained the gas stream is switched to another sorption vessel while the pressure in the first vessel is reduced to desorb the adsorbent component. A stripping step with inert (non-reactive) as or with treated product gas may then follow before the vessel is returned to the sorption portion of the cycle. Variants of the conventional PSA (cPSA) process have been developed including the partial pressure swing or displacement purge adsorption (PPSA), rapid cycle pressure swing adsorption (RCPSA), Dual Bed (or Duplex) PSA Process, and rapid cycle partial pressure swing or displacement purge adsorption (RCPPSA) technologies.
Temperature swing adsorption (TSA) provides an alternative to the pressure swing technology in which the sorbed component is desorbed by an increase in temperature typically achieved by the admission of high temperature gas, e.g., air, to the vessel in the regeneration phase. Rapid cycle thermal swing adsorption (RCTSA) is a variant of the conventional TSA process using short cycles, typically less than two minutes. TSA processes are generally available commercially from a number of technology suppliers, although the state of the art for large scale rapid cycle TSA units is considerably less advanced. Large scale slow (˜10 hr.) cycle internally heated TSA's have been used in natural gas processing for rigorous dehydration and mercaptan removal. In an internally heated thermal swing adsorption process, the gas or fluid used to heat the contactor directly contacts the adsorbent material. As such, the gas or fluid used to heat the contactor during regeneration can pass through the same channels that the feed gas does during the adsorption step. Externally heated thermal swing adsorption processes employ contactors having a separate set of channels to carry gases or fluids used to heat and cool the contactor so that gases used to heat and cool the contactor do not mix with the adsorbent that contacts the feed gas.
Indeed, adsorptive separation may be achieved, as noted by Yang by three mechanisms, steric, equilibrium, or kinetic: R. T. Yang, Gas Separation by Adsorption Processes, Imperial College Press, 1997, ISBN: 1860940471, ISBN-13: 9781860940477. A large majority of processes operate through the equilibrium adsorption of the gas mixture and kinetic separations have attracted considerable attention with the development of functional microporous adsorbents and efficient modeling tools. Relatively few steric separation processes have been commercialized. Kinetically based separation involves differences in the diffusion rates of different components of the gas mixture and allows different species to be separated regardless of similar equilibrium adsorption parameters. Kinetic separations utilize molecular sieves as the adsorbent since they exhibit a distribution of pore sizes which allow the different gaseous species to diffuse into the adsorbent at different rates while avoiding exclusion of any component of the mixture. Kinetic separations can be used for the separation of industrial gases, for example, for the separation of nitrogen from air and argon from other gases. In the case of the nitrogen/oxygen separation (for example, oxygen and nitrogen differ in size by only 0.02 nm), the separation is efficient since the rate of transport of oxygen into the carbon sieve pore structure is markedly higher than that of nitrogen. Hence, the kinetic separation works, even though the equilibrium loading levels of oxygen and nitrogen are virtually identical.
It has been reported that tapered adsorbent beds can improve product purity in PSA cycling applications. See James A. Ritter et al., Tapered Pressure Swing Adsorption Columns for Simultaneous Air Purification and Solvent Vapor Recovery, 37 IND. ENG. CHEM. RES. 2783-91 (1998). Ritter reports, among other things, that in an adiabatic bed in PSA cycling, product purity is several orders of magnitude higher in a tapered bed compared to a non-tapered bed. Ritter, at 2787, FIG. 5. This improvement in purity is of particular significance to hydrogen production where very high purities, e.g. 99.9% purity or greater are required. Moreover, Ritter describes another advantage of tapered versus conventional adsorbent beds with respect to temperature profile across the bed. Ritter, at 2787, FIG. 6. For tapered beds, the temperature profile at the end of the feed or adsorption step is substantially lower in the mass transfer zone. This lower temperature maintains a higher CO2 adsorption as well as a higher water gas shift conversion in the case of a syngas feedstream. Tapered adsorbent beds, however, are irregularly shaped and cannot be easily incorporated into a tight refinery environment, where space is at a premium.
It would therefore be advantageous to be able to incorporate the efficiencies gained via utilization of a tapered bed within a more conventionally shaped vessel.